Non-oriented electrical steel sheet and method for manufacturing the same

ABSTRACT

When a non-oriented electrical steel sheet is manufactured, simultaneously having superior magnetic properties and high strengths, a composition containing 0.02% or less of C, 4.5% or less of Si, 5.0% or less (including 0) of Ni, and 0.2% to 4.0% of Cu is used, and a solute Cu is allowed to appropriately remain in finish annealing. In the steel sheet thus obtained, finely shaped Cu is precipitated by aging treatment, and while the magnetic properties are not degraded, the yield stress is increased to not less than CYS (MPa) represented by the following formula:
 
note
 
 CYS =180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22 d   −1/2  
         where d is an average grain diameter (mm) of crystal grains.

TECHNICAL FIELD

The present invention relates to non-oriented electrical steel sheets,and more particularly, relates to a non-oriented electrical steel sheethaving high strengths and a low iron loss and a method for manufacturingthe same, the steel sheet being suitably used for a component receivinga large stress which is typically represented by a rotor for use in ahigh speed motor.

The non-oriented electrical steel sheet manufactured in accordance withthe present invention has a feature in which the yield strength and thelike are increased by aging treatment so that strengths of a rotorassembled therefrom are increased. In addition, the non-orientedelectrical steel sheet also has a feature in which since the yieldstrength is low before aging treatment, punching processing can beeasily performed.

BACKGROUND ART

In recent years, due to advancement of drive circuit systems of motors,it has become possible to perform frequency-control of drive powersupply, and as a result, a high speed motor driven by adjustable speedcontrol or driven at a higher frequency than a power frequency has beenincreasingly in demand. In the high speed motors driven as describedabove, it is necessary to use rotors having strengths capable ofwithstanding high speed rotation.

That is, a centrifugal force applied to a rotor is proportional to therotating-radius and is increased in proportional to the square of arotational speed. Hence, in medium-sized and large-sized high speedmotors, a stress more than 600 MPa may be applied to rotors thereof insome cases. Accordingly, for the high speed motors as described above,increase in strengths of the rotor must be achieved.

In addition, in view of recent improvement in motor efficiency, amagnet-embedded type (IPM: Interior Permanent Magnet) DC invertercontrol motor, in which permanent magnets are embedded in a rotor, hasalso been increasingly in demand. In the motor described above, magnetsembedded in the rotor are liable to jump out therefrom, and in order toprevent the magnets from jumping out, a large force is applied to anelectrical steel sheet used for the rotor. From this point of view, anelectrical steel sheet for use in the motor, in particular, for use inthe rotor has been required to have high strengths.

Since rotating devices such as motors and generators exploitelectromagnetic phenomena, core materials therefor are required to havemagnetic properties. In particular, the core materials preferably have alow iron loss and a high magnetic flux density.

In general, for assembling an iron core of a rotor, non-orientedelectrical steel sheets are formed by punching using a press machine andare then laminated to each other for the use. However, when a corematerial of rotors used for high speed motors cannot satisfy themechanical strengths described above, instead of that, a rotor made ofcast steel having higher strengths must be used. However, since the caststeel-made rotor mentioned above is a bulk product, compared to a rotorformed of electrical steel sheets laminated to each other, a ripple lossaffecting the rotor is large, thereby primarily causing decrease inmotor efficiency. The ripple loss indicates an eddy current loss causedby a high frequency magnetic flux.

Accordingly, an electrical steel sheet having superior magneticproperties and high strengths has been desired as a core material forrotors.

As a strengthening method from a metallurgical point of view, forexample, solid solution strengthening, precipitation strengthening, andgrain-refining strengthening have been known, and there are examples inwhich some methods mentioned above were applied to electrical steelsheets. For example, according to Japanese Unexamined Patent ApplicationPublication No. 60-238421, based on the results of investigation onadvantages and disadvantages of the each strengthening method mentionedabove, as a method having the least influence on magnetic properties,the use of solid solution strengthening has been proposed. In addition,a method has been disclosed in which, besides increase of the content ofSi to 3.5% to 7.0% (mass percent, hereinafter, the same as above), anelement having high capability of solid solution strengthening is added.

In addition, in Japanese Unexamined Patent Application Publication No.62-256917, a method for controlling the diameter of recrystallizedgrains has been disclosed in which the content of Si is set in the rangeof from 2.0% to 3.5%, the content of Ni or the contents of Ni and Mo areincreased, and low-temperature annealing at a temperature of 650 to 850°C. is performed. Furthermore, as a method using precipitationstrengthening, in Japanese Unexamined Patent Application Publication No.6-330255, a method has been disclosed in which the content of Si is setin the range of from 2.0% to 4.0% and fine carbides and nitrides of Nb,Zr, Ti, and/or V are precipitated.

By the methods described above, electrical steel sheets can be obtainedhaving a high strength to a certain extent. However, when steel is usedin which the contents of Si and an element for solid solutionstrengthening are high, as disclosed in Japanese Unexamined PatentApplication Publication No. 60-238421, cold rolling properties areextremely degraded, and as a result, it becomes disadvantageouslydifficult to perform stable industrial manufacturing. Furthermore, aproblem may arise in that magnetic flux density B₅₀ of the steel sheetobtained by this technique is also seriously decreased to 1.56 to 1.60T.

In the method disclosed in Japanese Unexamined Patent ApplicationPublication No. 62-256917, in order to increase the mechanicalstrengths, the growth of recrystallized grains must be suppressed bylow-temperature annealing, and as a result, in a relatively lowfrequency range, for example, of from a power frequency (approximately50 Hz) to several hundred Hertz, a problem occurs in that the iron lossis decreased.

Accordingly, the electrical steel sheet obtained by the method disclosedin Japanese Unexamined Patent Application Publication No. 62-256917cannot be used as a material for a stator member since the iron loss ofthis application is important in this frequency range. Hence, an extremedecrease in yield of the electrical steel sheet according to this methodcould not been avoided. That is, when stator and rotor members areobtained by punching, a ring-shaped stator member is generally punchedout from one steel sheet, and from a remaining central part of the samesteel sheet, a rotor member is also obtained by punching, therebyreducing waste. However, in the method disclosed in Japanese UnexaminedPatent Application Publication No. 62-256917, two types of members mustbe obtained from different steel sheets by punching, and as a result,the yield is unfavorably decreased.

On the other hand, according to the method disclosed in JapaneseUnexamined Patent Application Publication No. 6-330255, since thecarbides and nitrides themselves function as a barrier to magnetic wallmovement and interfere with the growth of crystal grains of anelectrical steel sheet, the degradation in iron loss is stilldisadvantageously large.

In addition, regardless of whether any of the methods described above isused, the electrical steel sheets manufactured thereby each have a highhardness, and as a result, the punchabilities thereof are inferior. Thatis, when the steel sheet for laminated core is punched out, die wearbecomes very large, and hence large burrs are liable to be generated inan early stage.

As will be described later, as one of the features of the presentinvention, the composition of a steel sheet according to the presentinvention contains a predetermined amount of Cu. Hence, apart from theproblems described above, the current status of Cu used in non-orientedelectrical steel sheets will be described.

As an example in which Cu is added to an electrical steel sheet, atechnique for improving punchabilities has been disclosed in JapaneseUnexamined Patent Application Publication No. 62-89816 in which 0.1 to1.0% of C is added to a steel sheet so as to precipitate graphite. As amethod of recrystallization annealing (finish annealing), box annealingis recommended. In this technique, as an element facilitating theprecipitation of graphite, Cu in an amount of 1.0% or less isrecommended to be added; however, disadvantage in cost is also implied.

However, the electrical steel sheet described above having a compositioncontaining 0.1% or more of C is an exceptional one, and in a generalelectrical steel sheet, the addition of Cu is not recommended in view ofthe magnetic properties and the like. For example, in JapaneseUnexamined Patent Application Publication No. 9-67654, a non-orientedelectrical steel sheet containing more than 1% to 3.5% of Si or the likehas been disclosed; however, since the precipitation of CuS and the likehas adverse influences on the magnetic properties, the content of Cu islimited to 0.05% or less.

In addition, as a technique which contain a larger amount of Cu thanthat described above, a method has been disclosed in Japanese UnexaminedPatent Application Publication No. 8-295936 in which a non-orientedelectrical steel sheet is manufactured from raw materials includingscrap steel. In this technique, in order to reduce adverse influences onthe magnetic properties caused by alloying elements (0.015% to 0.2% ofCu: 0.01% to 0.5% of Ni: 0.02% to 0.2% of Cr: 0.003% to 0.2% of Sn: andthe like) contained in scrap, for example, measures are proposed inwhich the contents of V and Nb are limited, and in which the diameter ofcrystal grains after hot-rolled sheet annealing is controlled to 50 μmor less. However, also for this technique, the above elements such as Cuare naturally disadvantageous, and a primary object of this technique isonly to reduce the adverse influences of the above elements. Inaddition, the contents of Cu and the like thus disclosed are small.

Furthermore, as steel which does not contain Si, high-strength steelused for electric machinery has been disclosed in Japanese UnexaminedPatent Application Publication No. 49-83613, the steel being composed of1% to 5% of Cu, 1% to 5% of Ni, and iron as the balance. According tothis technique, after solution treatment-quenching and cold rolling arerepeatedly performed for steel having the above composition, agingtreatment is performed, and then steel having a high strength and a lowiron loss can be obtained. However, degradation in iron loss caused byaging treatment has not been satisfactorily suppressed.

DISCLOSURE OF INVENTION

[Problems to be Solved by the Invention]

As described above, in order to stably perform industrial manufacturingof an electrical steel sheet which simultaneously has high strengths anda low iron loss, the conventional methods have not been satisfactory.

In addition, an object of sufficiently increasing rotor strengths whilesuperior punchabilities and a preferable iron loss are maintained hasnot been accomplished by the above conventional methods. In particular,it has been believed that since the punchabilities degrade as the yieldstrength is improved, superior punchabilities and high yield strengthcannot be simultaneously obtained.

An object of the present invention is to propose a non-orientedelectrical steel sheet capable of simultaneously satisfying superiormagnetic properties and high strengths and a method capable of stablyperforming industrial manufacturing of the steel sheet described above.

In addition, the present invention also proposes a non-orientedelectrical steel sheet capable of achieving an object in which rotorstrengths are sufficiently increased while superior punchabilities and apreferable iron loss are maintained and a method for manufacturing thesteel sheet described above.

[Means for Solving the Problems]

In order to achieve the above objects, the inventors of the presentinvention carried out various investigations focusing on anage-hardening phenomenon of steel containing Cu, and as a result, meansfor simultaneously obtaining a superior iron loss and high strengths wasfinally established.

That is, for example, as disclosed in Japanese Unexamined PatentApplication Publication No. 60-238421, it has been believed thatalthough strengths are increased, precipitates in steel suppress themagnetic wall movement and also degrade the iron loss (hysteresis loss).In addition, according to the finding that was first discovered by theinventors of the present invention, particularly in Si-containing steel,Cu precipitates are liable to be grown large and coarse, and as aresult, it is difficult to avoid degradation in iron loss.

However, in spite of the conventional knowledge and the novel findingdescribed above, the inventors of the present invention newly found thatwhen an appropriate amount of Cu is added to steel, followed by agingtreatment, very fine Cu particles having an average particle diameter of1 nm to 20 nm can be uniformly precipitated in crystal grain interior,and that the very fine precipitates thus obtained are very effective forimprovement in strength, and in addition, do not substantially degradethe iron loss (hysteresis loss).

Furthermore, it was also found that, as for this Cu precipitation, whenCu and Ni are added in combination, since the amount of precipitatesgenerated in heat treatment in steel sheet manufacturing is remarkablyreduced, high strengths and a low iron loss can be stably obtained evenunder wide annealing conditions. Accordingly, the present invention wasfinally made.

In addition, the inventors of the present invention also succeeded informing an electrical steel sheet which can impart high strengths to arotor or the like assembled therefrom while having superiorpunchabilities. That is, before a punching step, an electrical steelsheet which is not processed by aging treatment and which has a lowyield strength is prepared, and aging treatment is performed right afterthe punching step or after a rotor or the like is assembled, therebyimproving strengths of a laminated core assembled from the above steelsheet.

The aspects of the present invention are as follows.

(1) A high-strength non-oriented electrical steel sheet having superiormagnetic properties, comprises: on a mass percent basis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: 5% or less (including 0%); and

Cu: 0.2% to 4%,

wherein the yield stress is not less than CYS (MPa) represented by thefollowing formula 1:noteCYS=180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22d^(−1/2)  (Formula 1)

where d is an average grain diameter (mm) of crystal grains.

(2) A high-strength non-oriented electrical steel sheet having superiormagnetic properties, comprises: on a mass percent basis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: 5% or less (including 0%); and

Cu: 0.2% to 4%,

wherein a volume ratio of Cu precipitates in crystal grain interior isin the range of from 0.2% to 2%, and

an average particle size of the Cu precipitates is in the range of from1 to 20 nm.

The average particle size of the Cu precipitates is obtained as asphere-base diameter by calculation. Hereinafter, the average particlesize will be represented in the same manner as described above.

(3) In the high-strength non-oriented electrical steel sheet havingsuperior magnetic properties, according to the above (1), a volume ratioof Cu precipitates in the crystal grains is in the range of from 0.2% to2%, and an average particle size of the Cu precipitates is in the rangeof from 1 to 20 nm.

(4) An age-hardenable non-oriented electrical steel sheet havingsuperior punchabilities and magnetic properties (iron loss), comprises:on a mass percent basis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: 5% or less (including 0%); and

Cu: 0.2% to 4%,

Wherein, after aging treatment is performed at 500° C. for 10 hours, theyield stress of the steel sheet is not less than CYS (MPa) representedby the following formula 1:noteCYS=180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22d^(−1/2)  (Formula 1)

where d is an average grain diameter (mm) of crystal grains.

(5) The non-oriented electrical steel sheet according to one of theabove (1) to (4), further comprises at least one of Zr, V, Sb, Sn, Ge,B, Ca, a rare earth element, and Co as a component,

wherein the content of each of Zr and V is 0.1% to 3%,

the content of each of Sb, Sn, and Ge is 0.002% to 0.5%,

the content of each of B, Ca, and the rare earth element is 0.001% to0.01%, and

the content of Co is 0.2% to 5%

(according to the above (1) to (3), the high-strength non-orientedelectrical steel sheet having superior magnetic properties is obtained,and according to the above (4), the age-hardenable non-orientedelectrical steel sheet having superior punchabilities and magneticproperties is obtained).

Instead of the CYS requirement, the non-oriented electrical steel sheetaccording to one of the above (1) to (5) may satisfy requirement inwhich the tensile strength is not less than CTS (MPa) represented by thefollowing formula 2:noteCTS=5,600[% C]+87[% Si]+15[% Mn]+70[% Al]+430[% P]+37[% Ni]+22d^(−1/2)+230  (Formula 3)

where d is an average grain diameter (mm) of crystal grains.

In the individual inventions described above, the balance of thecomposition of the steel sheet is preferably composed of Fe andinevitable impurities.

In addition, in the individual inventions described above and preferableembodiments, Ni in an amount of 0.5% or more is preferably contained,and this Ni content is significantly preferable when the CTS is definedas the requirement.

(6) A method for manufacturing a high-strength non-oriented electricalsteel sheet having superior magnetic properties, comprises the steps of:

performing hot rolling of a steel slab containing on a mass percentbasis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: less than 0.5% (including 0%); and

Cu: 0.2% to 4%,

then performing cold rolling or warm rolling to obtain a rolled steelsheet having a final sheet thickness,

then performing finish annealing in which heating is performed to a Cusolid solution temperature (temperature of forming a Cu solidsolution)+10° C. or more, followed by cooling in which a cooling rate ina temperature range of from the Cu solid solution temperature to 400° C.is 10° C./s or more; and

subsequently performing aging treatment at a temperature in the range offrom 400 to 650° C.

(7) A method for manufacturing a high-strength non-oriented electricalsteel sheet having superior magnetic properties, comprises the steps of:

performing hot rolling of a steel slab containing on a mass percentbasis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: 0.5% to 5%; and

Cu: 0.2% to 4%,

then performing cold rolling or warm rolling to obtain a rolled steelsheet having a final sheet thickness,

then performing finish annealing in which heating is performed to a Cusolid solution temperature+10° C. or more, followed by cooling in whicha cooling rate in a temperature range of from the Cu solid solutiontemperature to 400° C. is 1° C./s or more; and

subsequently performing aging treatment at a temperature in the range offrom 400 to 650° C.

(8) In the method for manufacturing a high-strength non-orientedelectrical steel sheet, according to the above (6) or (7), instead ofthe “Cu solid solution temperature”, Ts (° C.) represented by thefollowing formula 2 is used.NoteTs(° C.)=3,351/(3.279−log₁₀[% C])−273  (Formula 2)

(9) In the method for manufacturing a high-strength non-orientedelectrical steel sheet having superior magnetic properties, according toone of the above (6) to (8), the steel slab further contains at leastone of Zr, V, Sb, Sn, Ge, B, Ca, a rare earth element, and Co,

wherein the content of each of Zr and V is 0.1% to 3%,

the content of each of Sb, Sn, and Ge is 0.002% to 0.5%,

the content of each of B, Ca, and the rare earth element is 0.001% to0.01%, and

the content of Co is 0.2% to 5%.

In addition, the compositions of the inventions according to the above(6) to (9) may be described in a different manner as follows.

That is, in the case in which the steel slab composition described abovecontains Ni in an amount of 5% or less (including zero; that is, thecase is included in which addition is not performed), when the coolingrate in finish annealing is set to 10° C./s or more in a temperaturerange of from the Cu solid solution temperature or Ts to 400° C., theobject of the present invention can be achieved. Furthermore,particularly in the case in which the content of Ni is in the range offrom 0.5% to 5%, even if the cooling rate described above is not limitedto 10° C./s or more, the object of the present invention can be achievedas long as the cooling rate is set to 1° C./s or more. Of course, evenwhen the cooling rate is set to 10° C./s or more, it is effective thatNi in an amount of 0.5% or more be contained.

(10) A method for manufacturing an age-hardenable non-orientedelectrical steel sheet having superior punchabilities and magneticproperties, comprises the steps of:

performing hot rolling of a steel slab containing on a mass percentbasis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: less than 0.5% (including 0%); and

Cu: 0.2% to 4%,

then performing cold rolling or warm rolling to obtain a rolled steelsheet having a final sheet thickness; and

then performing finish annealing in which heating is performed to a Cusolid solution temperature+10° C. or more, followed by cooling in whicha cooling rate in a temperature range of from the Cu solid solutiontemperature to 400° C. is 10° C./s or more.

(11) A method for manufacturing an age-hardenable non-orientedelectrical steel sheet having superior punchabilities and magneticproperties, comprises the steps of:

performing hot rolling of a steel slab containing on a mass percentbasis,

C: 0.02% or less (including 0%);

Si: 4.5% or less;

Mn: 3% or less;

Al: 3% or less;

P: 0.5% or less (including 0%);

Ni: 0.5% to 5%; and

Cu: 0.2% to 4%,

then performing cold rolling or warm rolling to obtain a rolled steelsheet having a final sheet thickness, and

then performing finish annealing in which heating is performed to a Cusolid solution temperature+10° C. or more, followed by cooling in whicha cooling rate in a temperature range of from the Cu solid solutiontemperature to 400° C. is 1° C./s or more.

(12) In the method for manufacturing an age-hardenable non-orientedelectrical steel sheet, according to the above (10) or (11), instead ofthe “Cu solid solution temperature”, Ts (° C.) represented by thefollowing formula 2 is used:noteTs(° C.)=3,351/(3.279−log₁₀[% C])−273  (Formula 2).

(13) In the method for manufacturing an age-hardenable non-orientedelectrical steel sheet having superior punchabilities and magneticproperties, according to any one of the above (10) to (12), the steelslab further contains at least one of Zr, V, Sb, Sn, Ge, B, Ca, a rareearth element, and Co,

wherein the content of each of Zr and V is 0.1% to 3%,

the content of each of Sb, Sn, and Ge is 0.002% to 0.5%,

the content of each of B, Ca, and the rare earth element is 0.001% to0.01%, and

the content of Co is 0.2% to 5%.

In the inventions according to the above (10) to (13), the age-hardeningtreatment described in the inventions according to the above (6) to (9)is not included. The reason for this is based on the concept in that,for example, the age-hardening treatment may be performed at a customersite in a process for manufacturing laminated magnetic cores and thelike. However, the present invention described above is not limited tothe use described above.

The invention according to the above (4) is also based on the sameconcept as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a dark field image of precipitated Cu particles observed usinga scanning transmission electron microscope (STEM), in which the Cuparticles are obtained by finish annealing of 1.8% Si-1.0% Cu steel,followed by aging treatment at 500° C. for 8 hours.

FIG. 2 is a graph showing the influence of a cooling rate in finishannealing on an iron loss obtained after aging treatment.

FIG. 3 is a graph showing the influence of a cooling rate in finishannealing on the tensile strength obtained after aging treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, each of the elements of the present invention will be described indetail.

[Composition of Steel Sheet]

First, the ranges of individual components of the composition and thereasons of limitation thereof will be described. In the presentinvention, % used for indicating the steel composition is mass percentunless otherwise specifically stated.

C: 0.02% or Less

When the content of C is more than 0.02%, the iron loss is extremelydegraded by magnetic aging, and hence the content is limited to 0.02% orless. Alternatively, when the content is preferably set to 0.01% or lessor 0.005% or less, and is more preferably set to 0.003% or less, thedegradation in iron loss caused by magnetic aging can be decreased toapproximately zero.

In addition, it may be C-free, that is, the content may be 0%; however,in general, 0.0005% or more of C is contained.

Si: 4.5% or Less

While being a useful deoxidizing agent, Si has a considerable effect ofreducing the iron loss of an electrical steel sheet since the electricresistance is increased. Furthermore, improvement in strength isperformed by solid solution strengthening. As a deoxidizing agent, whenthe content is 0.05% or more, the effect becomes significant. Forreduction in iron loss and for solid solution strengthening, the contentis set to 0.5% or more and is more preferably set to 1.2% or more.However, when the content is more than 4.5%, degradation in rollingproperties of steel sheets becomes serious, and hence the content islimited to 4.5% or less. More preferably, the content is limited to 4.2%or less.

Mn: 3% or Less

While being a useful element for improving strengths by solid solutionstrengthening, Mn is also a useful element for improving hotbrittleness, and the content is preferably set to 0.05% or more.However, excessive addition causes degradation in iron loss, and hencethe content is limited to 3% or less. In addition, the content may beset to 3.0% or less. The content of Mn is more preferably 2.0% or less,even more preferably 0.1% to 1.5%, and still even more preferably 1.0%or less.

Al: 3% or Less

Al is a useful element as a deoxidizing agent and is also useful forimproving the iron loss. The content of Al is preferably set to 0.5 ppmor more and more preferably set to 0.1% or more. However, excessiveaddition causes degradation in rolling properties or degradation inpunchabilities, and hence the content is preferably set to 3% or less.In addition, the content may be set to 3.0% or less.

However, when the content is 4.0% or less, since the degradation inrolling properties is not so significant, for example, in application inwhich punching processing is performed before age-hardening treatment,the upper limit may be set to 4.0%.

In addition, the content is more preferably set to 2.5% or less.

P: 0.5% or Less

Since remarkable capability of solid solution strengthening can beobtained by addition of a relatively small amount of P, P is a veryuseful element for improving strengths, and the content thereof ispreferably set to 0.01% or more. On the other hand, since excessiveaddition may cause embrittlement due to segregation, grain boundarycracking and degradation in rolling properties occur, and hence thecontent is set to 0.5% or less. In addition, the content may be set to0.50% or less. The content is more preferably 0.2% or less.

On the other hand, when the content of P is positively decreased, thehot and cold rolling properties can be improved. From this point ofview, the content of P may be less than 0.01%. In this case, when it ispossible, it may be P-free, that is, the content may be 0%; however,since P is inevitably contained in iron ore or molten iron as animpurity, the content is decreased by dephosphorization treatment in amanufacturing process. A decreased amount of P may be determined inaccordance with dephosphorization treatment conditions, treatment cost,and the like, and in general, the lower limit of the content of P isapproximately 0.005%.

Cu: 0.2% to 4%

When fine Cu precipitates are formed by aging treatment, the strengthsare significantly increased without any substantial degradation in ironloss (hysteresis loss). In order to obtain the effect described above,the content must be 0.2% or more. That is, when the content is less than0.2%, even when the other structural requirements (composition,manufacturing conditions, and the like) of the present invention are allsatisfied, a sufficient precipitate amount cannot be obtained. On theother hand, when the content is more than 4%, since large and coarseprecipitates are formed, in addition to considerable degradation in ironloss, increase of strengths is reduced. Accordingly, the content of Cuis set in the range of from 0.2% to 4%. In addition, the upper limit maybe set to 4.0% or less.

The preferable lower limit is 0.3% and more preferable lower limit is0.5%, 0.7%, or 0.8%. In particular, when the content is 0.5% of more,strengthening can be stably obtained.

In addition, the preferable upper limit is 3.0% or less, and morepreferably, the upper limit is 2.0% or less.

Ni: 5% or Less

Ni is not an essential element, and the lower limit may be 0%, that is,it may be Ni-free. In addition, even when a small amount of Ni iscontained as an inevitable impurity, any problem may not occur.

However, since Ni is a useful element for improving strengths by solidsolution strengthening and for improving magnetic properties, thecontent is preferably set to 0.1% or more.

In addition, when being added to Cu-containing steel as described in thepresent invention, Ni has an influence on the solid solution state andthe precipitation state of Cu and has an effect of stably forming veryfine Cu precipitates by aging. That is, in Si-containing steel, inparticular, in high Si-containing steel, the growth of Cu precipitatesis likely to be facilitated, and due to this phenomenon, it has beenbelieved that insufficient age hardening and degradation in magneticproperties are liable to occur. However, when Ni is present, theformation of large and coarse Cu precipitates is suppressed, and hencethe effect of improving the capability of precipitation strengthening byaging can be easily obtained. As a result, the effect of improvingstrengths by Cu precipitation by aging can be significantly improved, orthe range of required process conditions can be widened. In order toobtain this effect, the content is very preferably set to 0.5% or more.

Furthermore, Ni has an effect of decreasing the number of surfacedefects of hot-rolled steel sheets, called scab (sliver), therebyincreasing the yield of steel sheets. The effect described above can beobtained when the content is set to 0.1% or more; however, as isexpected, the content is preferably set to 0.5% or more.

However, when the content is more than 5%, the various effects describedabove are saturated, and the cost is unnecessarily increased; hence, theupper limit is set to 5%. In addition, the upper limit may be set to5.0%. A more preferable upper limit is 3.5%, and even more preferableupper limit is 3.0%.

In addition, in order to obtain the various effects described above, amore preferable lower limit is 1.0%.

The basic composition of the non-oriented electrical steel sheet of thepresent invention is as described above, and in addition to the abovecomponents, known elements for improving magnetic properties, that is,Zr, V, Sb, Sn, Ge, B, Ca, a rare earth element, and Co, may also beadded alone or in combination. However, the content thereof must becontrolled so as not to degrade the object of the present invention. Inparticular,

as for Zr and V, the content is 0.1% to 3%, or 0.1% to 3.0%, andpreferably 0.1 to 2.0%.

As for Sb, Sn, and Ge,

the content is 0.002% to 0.5%, preferably 0.005% to 0.5%, and morepreferably 0.01 to 0.5%.

As for Ba, Ca, and a rare earth element, the content is 0.001% to 0.01%.

As for Co, the content is 0.2% to 5%, or 0.2% to 5.0%, and preferably0.2 to 3.0%.

Since Co has a slightly higher strengthening capability, for example, inapplication in which punching processing is performed beforeage-hardening treatment, elements described above other than Co, thatis, Zr, V, Sb, Sn, Ge, B, Ca, and a rare earth element, are preferablyused alone or in combination. Since also being categorized as an elementfor improving magnetic properties, Ni may be included in the groupdescribed above; however, the effect of Ni is remarkable as compared tothat of the elements described above, Ni is separately described.

As elements other than the elements described above, Fe (iron) andinevitable impurities are preferably mentioned. As for S and N as aninevitable impurity, the content thereof is preferably set toapproximately 0.01% or less in view of iron loss.

In particular, when a residual amount of S is large, since a CuSprecipitate is formed, grain growth in finish annealing is suppressed,thereby degrading the iron loss. Accordingly, the S content ispreferably set to at most approximately 0.02%.

As another inevitable impurity, O may be mentioned, and the contentthereof is set to approximately 0.02% or less and preferably set to0.01% or less.

In addition, as inevitable impurities in a broader sense, for example,there are mentioned Nb, Ti, and Cr, which may be contained in some casesdue to manufacturing reasons, and the contents thereof are preferablyset to approximately 0.005% or less, 0.005% or less, 0.5% or less,respectively.

[Steel Sheet and Cu Precipitates]

The subject of the present invention is basically a non-orientedelectrical steel sheet regardless of whether it is processed byage-hardening treatment or not. Although being a ferrite single phasesteel in general, the non-oriented electrical steel sheet has variouscompositions and textures, and they are not specifically limited. Thecomposition and texture may also be freely designed within the scope ofthe present invention; however, the iron loss value is preferably small,and W₁₅/W₅₀ is preferably set to approximately 6 W/kg or less.

In addition, Cu precipitates which will be described below aresubstantially composed of Cu alone; however, when very fine precipitatesare formed, Fe in a solid solution form may be contained in Cuprecipitates. The Cu precipitates also include the precipitates asdescribed above.

In some cases, depending on manufacturing conditions, large and coarseCu precipitates may be observed in grain boundaries; however, as for theamount of precipitates and the average particle size thereof, theprecipitates in grains, which practically contribute to thestrengthening, are only regarded as the precipitates described above.

[Texture and Properties of Steel Sheet before Age-Hardening Treatment]

In the non-oriented electrical steel sheet of the present inventionbefore age-hardening treatment, it is important that Cu in the steelsheet be present as the solute Cu in a sufficient amount in the steel.When a large amount of fine Cu precipitates is already present beforeaging treatment, the punchabilities are not only be degraded due to theincrease in hardness but also the increase in yield strength by agingtreatment performed after punching becomes small. On the other hand,when large Cu precipitates are present in a matrix of crystal grainbefore aging treatment, besides the deterioration in iron loss,precipitation of Cu during aging treatment occurs on precedent coarse Cuprecipitates as nucleuses, and hence larger and coarser Cu precipitatesare further formed. As a result, the iron loss is further seriouslydeteriorated thereby.

When steel is used in which 0.20% to 4.0% or preferably 0.5% to 2.0% ofCu is contained, by aging treatment at 500° C. for 10 hours, fine Cuprecipitates having an average particle size of approximately 5 nm canbe formed in steel. In more particular, fine Cu precipitates having anaverage particle size of approximately 1 nm to 20 nm, the averageparticle size of the Cu precipitates being obtained as a sphere-basediameter, can be precipitated at a volume ratio of 0.2% to 2% withrespect to the entire steel sheet. The detail will be explained indescription about the steel sheet after aging.

As for the solute Cu before aging, the amount thereof is preferably 0.2%or more and more preferably 0.4% or more, 0.5% or more, or 0.8% or more.The upper limit of the solute Cu is naturally the content of Cu insteel, and the maximum amount of the solute Cu is equal to the maximumcontent of Cu.

According to the formation of fine Cu precipitates described above, theyield stress can be increased by at least 100 MPa and by approximately150 MPa under preferable conditions. In particular, when the Cu contentis in an optimum range, such as in the range of from 0.5% to 2.0%, orpreferably in the range of from 0.7% (0.8% or more is more suitable) to2.0%, the yield stress can be increased by 150 to 250 MPa.

According to the increase in strength as described above, yield stressYS (MPa) obtained after aging is preferably not less than CYSrepresented by the following formula 1.CYS=180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22d^(−1/2)  (Formula 1)

In this formula, the coefficient of the term of each element indicatesthe amount of solid solution strengthening per 1% of each element, and dindicates the average crystal grain diameter (diameter: mm). Themeasurement method of d is performed as follows. A cross section of asample is etched by a nital etchant or the like, the cross section beingin the thickness direction along a rolling direction (a so-calledrolling-direction cross section), and is then observed by an opticalmicroscope. Subsequently, the average area of crystal grains iscalculated from the observation field area and the number of crystalgrains in the field. Next, d is defined as a circle-base diametercalculated based on the area of the crystal grains.

As the average crystal grain diameter d is decreased, higher strengthcan be obtained; however, the iron loss is degraded. Accordingly, inaccordance with desired strengths and iron loss properties, the crystalgrain diameter d is adjusted. Although depending on a desired iron losslevel, an appropriate crystal grain diameter is generally approximately20 to 200 μm.

By the strengthening as described above, for example, the yield stressof a laminated sheet formed into a rotor core can be increased to 450MPa or more. The increase in yield strength by the mechanism describedabove will not cause any considerable degradation in iron loss (increasein iron loss value). For example, the amount of degradation in iron lossrepresented by W₁₅/W₅₀ is 1.5 W/kg or less, and when the Cu amount isrelatively small, such as 3% or less, the amount described above ismerely 1.0 W/kg or less.

In addition, when the non-oriented electrical steel sheet of the presentinvention before the age-hardening treatment is processed byage-hardening treatment, the tensile strength (TS) (MPa) is preferablyincreased to not less than CTS represented by the following formula 3.The requirement described above can be approximately obtained whenappropriate Cu precipitation after aging is performed by controlling thecomposition range and the states of solid solution and precipitation ofCu as described above:CTS=5,600[% C]+87[% Si]+15[% Mn]+70[% Al]+430[% P]+37[% Ni]+22d^(−1/2)+230  (Formula 3).

The meanings of the terms of the above formula are the same as thosedescribed in the formula 1 except that each of the terms relates to thetensile strength.

[Structure and Properties of Steel Sheet after Age-Hardening Treatment]

In the non-oriented electrical steel sheet of the present inventionafter age hardening treatment, it is important that Cu in the steelsheet be finely precipitated in steel. Even when the solute Cu(non-precipitated state) is present, higher strengths cannot beachieved. On the contrary, Cu precipitates, which are not finely formedin a predetermined dimensional range, not only degrade the iron loss butalso have small contribution to the strengthening. Hence, it isimportant that without degrading the iron loss, Cu be allowed to bepresent as fine precipitates which are finely formed in a predetermineddimensional range so as to contribute to the strengthening.

As described above, a preferable Cu precipitation state is that Cuprecipitates having an average particle size, which is the sphere-basediameter described above, in the range of from 1 to 20 nm are formed incrystal grain interior at a volume ratio of 0.2% to 2% with respect tothe entire steel sheet. In addition, the particle size of Cuprecipitates is preferably approximately 20 nm or less.

In general, when the volume ratio of Cu precipitates is high and theaverage particle size thereof is small, the average distance betweenparticles is decreased. Hence, the increase in strength by aging becomeslarge. However, although the volume ratio is high, when the averageparticle size is large, significant increase in strength cannot beexpected, and on the contrary, suppression of magnetic wall movement mayoccur by large and coarse particles in some cases. A volume ratio whichcan stably realize sufficient strengthening is preferably in the rangeof from approximately 0.2% to 2%. In addition, the average particlesize, which is the sphere-base diameter described above, is preferablyin the range of from approximately 1 nm to 20 nm.

In investigations carried out by the inventors of the present invention,the average particle size (the sphere-base diameter described above) ofCu precipitates and the volume ratio thereof were obtained by thefollowing measurements and the statistical work. However, as long as thesame result can be obtained in a theoretical point of view, anothermethod may be used in stead of the following methods.

After several images (dark field images) of a sample in a region ofapproximately 400 by 400 (nm)² were photographed using a scanningtransmission electron microscope, the thickness of the sample beingmeasured beforehand, precipitated Cu particles were recognized by imageprocessing and, from the exterior appearance of each particle, acircle-base diameter thereof was also obtained by calculation.Subsequently, assuming that the diameter thus obtained represented thespherical-base diameter of each particle, the volume of each particlewas determined.

The recognition whether an observed particle was a Cu precipitate or notwas performed using an energy dispersive X-ray spectrometer (EDX)provided for the scanning transmission electron microscope.Specifically, a precipitate phase was irradiated with electron beamshaving a diameter of 1 nm or less, and compared to a surrounding matrixphase, the state in which Cu is apparently concentrated was confirmed bythe EDX spectrum thus obtained.

From individual particles obtained by image recognition, the volumethereof were calculated based on the assumption in that each particlehad a spherical shape, thereby obtaining the sum of the particlevolumes. Next, the sum of the particle volumes was divided by the numberof the particles, so that the average volume was obtained. From thisaverage volume, the sphere-base diameter was reversely obtained bycalculation, thereby obtaining the average particle size describedabove. In this measurement, all precipitate Cu particles in each fieldwere measured, and the number of fields was determined so as to measureat least 10 particles.

In order to obtain the average particle size, an evaluation method usinga so-called circle-base diameter may be used in which the circle-basediameters of individual particles, which were obtained by theobservation described above, are simply arithmetically averaged. In thepresent invention, as the particle size, the sphere-base diameterdescribed above is used; however, since having a value close to that ofthe diameter described above, the circle-base diameter may be used for atemporary evaluation.

In this measurement, when the observation region was too thin,precipitated particles may be allowed to fall more frequently, and whenthe region is too thick, precipitated particles in the image of ascanning transmission electron microscope become difficult to recognize;hence, the thickness of the observation region was set in the range offrom 30 to 60 nm. In addition, a sample formed from Cu-containing steelfor measurement by a scanning transmission electron microscope isgenerally electrodeposited with Cu atoms on the surface, and by theinfluence thereof, the amount of precipitates tends to be overestimated.In order to avoid this influence, in the observation, a sample processedby surface cleaning treatment using argon ions was used. FIG. 1 shows anexample of a dark field image of a steel sheet containing 1.8% of Si and1.0% of Cu processed by aging, according to the present invention,photographed by using a scanning transmission electron microscope.Particles shining white are Cu precipitated by the aging.

In addition, as described above, the measurement of the amount ofprecipitates and the average particle size were performed only forprecipitates present inside grains.

In addition, finer Cu precipitates further contribute to strengthening;however, when the particle size of Cu in steel is less thanapproximately 1 nm, the effect of increasing strengths is saturated, andin addition, it becomes difficult to perform measurement using ascanning transmission electron microscope. Accordingly, in some cases,such severe product control becomes difficult. Hence, in view ofindustrial manufacturing, the average particle size is preferablycontrolled in the range of approximately 1 nm or more.

On the other hand, when the average particle size is more thanapproximately 20 nm, the contribution to strengthening is decreased, andin addition, degradation in iron loss tends to increase; hence, theaverage particle size is preferably limited to not more thanapproximately 20 nm.

In addition, the yield stress YS (MPa) of the steel sheet of the presentinvention after age-hardening treatment is preferably not less than CYSrepresented by the following formula 1.CYS=180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22d^(−1/2)  (Formula 1)

In addition, the tensile strength TS (MPa) of the steel sheet of thepresent invention after age-hardening treatment is preferably not lessthan CTS represented by the following formula 3:CTS=5,600[% C]+87[% Si]+15[% Mn]+70[% Al]+430[% P]+37[% Ni]+22d^(−1/2)+230  (Formula 3).[Manufacturing Method]

In order to manufacture a high-strength non-oriented electrical steelsheet having a superior iron loss, according to the present invention,first, steel melted to have the predetermined composition describedabove by a converter or an electric furnace is formed into a steel slabthrough continuous casting or blooming rolling following ingotformation. The composition of the steel slab may be the same as that ofa targeted product steel sheet.

Next, the slab thus obtained is hot-rolled and is then processed byhot-rolled sheet annealing whenever necessary.

The hot-rolled steel sheet thus obtained (or hot-rolled annealed steelsheet) is processed by cold rolling once or at least two cold rollingincluding intermediate annealing to obtain a sheet having a productthickness. In this step, instead of at least one cold rolling step, warmrolling may be performed. The above sequential steps are described byway of example, and the point is to obtain a steel sheet having thecomposition described above and a predetermined thickness as the sheetproduct through appropriate casting and processing steps. For example,the following process may be carried out in which casting is performedto form a sheet having a thickness approximately equivalent to that of acommon hot-rolled steel sheet, followed by heat treatment whenevernecessary, and in addition, cold rolling or warm rolling may then beperformed.

According to the present invention, since strengthening is performed ina subsequent step without increasing the Si amount of a startingmaterial, manufacturing can be performed by cold rolling instead of warmrolling. However, since having effects of improving texture and ofimproving an iron loss and a magnetic flux density, warm rolling may beused.

In addition, at least before final cold rolling (or before warm rolling;hereinafter, the same as above), means for preventing large and coarseCu precipitates from remaining is preferably taken in order to obtainstable aging properties. When a great amount of large and coarse Cuprecipitates remains before the final cold rolling, in a final annealingstep which is subsequently performed, a treatment time for reliablyturning the large and coarse Cu precipitates into a solid solution formis increased.

As the treatment for preventing large and coarse Cu precipitates fromremaining, for example, a method may be mentioned in which a coilingtemperature in hot rolling is set to approximately 600° C. or less andpreferably set to approximately 550° C. or less.

As another method, a method may be mentioned in which after hot rollingand before final cold rolling, annealing such as hot-rolled sheetannealing or intermediate annealing is performed under predeterminedconditions. In this annealing, the large and coarse Cu precipitates areturned into a solid solution form by heating to a Cu solid solutiontemperature+approximately 10° C. or more, followed by cooling in which acooling rate in the range of from the Cu solid solution temperature to400° C. is approximately 5° C./s or more.

As the Cu solid solution temperature, a temperature at which Cu in steelis substantially and sufficiently turned into a solid solution form maybe calculated from thermodynamic data, or the temperature may beconfirmed by experiments whether Cu in steel is substantially turnedinto a solid solution form.

As one example, according to “Das Kupfer-Eisen Zustandsdiagramm imBereich von 650 bis 1,050° C.” (G. Salje and M. Feller-Kniepmeier; Z.Metallkde, 69 (1978) pp. 167 to 169), the Cu solid solution temperaturecan be approximately obtained by the following formula 2.Ts(° C.)=3,351/(3.279−log₁₀[% C])−273  (Formula 2)Accordingly, in the hot-rolled sheet annealing described above, afterheating is performed to Ts+approximately 10° C. or more, cooling may beperformed at a rate of approximately 5° C./s or more in the range offrom Ts to 400° C. In this formula, [% Cu] indicates the content of Cuin steel on a mass percent basis.

The cooling rate indicates an average cooling rate in the temperaturerange described above.

When the annealing treatment is performed under the above conditions, acoiling temperature in hot rolling is not specifically limited. Ofcourse, while the coiling temperature is set to approximately 600° C. orless and preferably approximately 550° C. or less, the annealingtreatment described above may also be performed.

As the annealing treatment, in general, hot-rolled sheet annealing canbe advantageously performed in terms of cost. In addition, afterhot-rolled sheet annealing is performed under the conditions describedabove, intermediate annealing may be performed under the conditionssimilar to those of the above hot-rolled sheet annealing so that thelarge and coarse Cu precipitates are reliably turned into a solidsolution form.

For the steel sheet having a product sheet thickness processed by coldrolling, warm rolling, or the like, finish annealing is performed.Furthermore, after the finish annealing, whenever necessary, aninsulating film is applied, dried, and baked.

In addition, whenever necessary, component adjusting treatment such asdecarburization annealing, silicon deposition, or the like may beperformed, for example, before finish annealing.

In order to turn Cu into a solid solution form in the finish annealingdescribed above, the annealing temperature is set to {a Cu solidsolution temperature+approximately 10° C.} or more. When the annealingtemperature is less than (a Cu solid solution temperature+approximately10° C.), large and coarse Cu precipitates present before annealing andCu precipitates which are formed in a process of the finish annealingremain in a product, and as a result, the iron loss is degraded. Inaddition, in subsequent aging annealing, since solute Cu is consumed forthe growth of the large and coarse Cu precipitates, the amount of thesolute Cu itself also becomes insufficient, and hence high strengthscannot be obtained by age-hardening.

Instead of a practical Cu solid solution temperature, for example, Tsobtained by the following approximate formula 2 can be used as describedabove.Ts(° C.)=3,351/(3.279−log₁₀[% C])−273  (Formula 2)

When Cu is only contained and Ni is not contained, in particular, in thecase of a steel sheet containing less than 0.5% of Ni (including 0), inorder to suppress the precipitation of Cu in a cooling step of finishannealing, cooling is performed at a rate of approximately 10° C./s ormore from the Cu solid solution temperature (or Ts) to 400° C. Inaddition, in a temperature range of from an annealing temperature or900° C. (whichever is lower) to 400° C., the cooling rate is alsopreferably set to approximately 10° C./s or more.

When the cooling rate is less than approximately 10° C./s, since Cu isalso precipitated in a large and coarse form, the iron loss is degraded,and in addition, even in subsequent age-hardening, sufficient increasein strength cannot be obtained. In addition, due to re-precipitation ofCu, the yield strength is increased, and hence the punchabilities aredegraded.

On the other hand, in the case in which 0.5% or more of Ni is containedtogether with Cu, when the cooling rate in the temperature rangedescribed above is approximately 1° C./s or more, formation of large andcoarse precipitates can be suppressed in cooling, and in subsequentaging treatment, without causing considerable degradation in iron loss,sufficient increase in strength can be obtained. In addition, since thestrength before aging treatment can be maintained small, thepunchabilities are also superior. That is, when aging treatment isperformed for steel containing both Cu and Ni, compared to the case inwhich Ni is not contained, stable properties can be obtained under morevarious finish annealing conditions.

Accordingly, in a steel composition containing 0.5% or more of Ni, in acooling step of finish annealing, the cooling rate in the temperaturerange of from the Cu solid solution temperature (or Ts) to 400° C. isset to approximately 1° C./s or more. In addition, in the temperaturerange of from the annealing temperature or 900° C. (whichever is lower)to 400° C., the cooling rate is also preferably set to approximately 1°C./s or more.

In the present invention, it is preferable that a steel texture afterfinish annealing be substantially a ferrite single phase. Whenmartensite transformation or the like occurs in part of the textureduring cooling, due to fine crystal texture formation or residual straingenerated in the transformation, the magnetic properties are degraded.It is difficult to totally eliminate the adverse influences describedabove in subsequent age-heating treatment.

In order to make a steel texture into a ferrite single phase, in coolingin the temperature range of from the Cu solid solution temperature (orTs) to 400° C., excessively rapid cooling is preferably avoided.Although a particular cooling rate depends on the steel texture, ingeneral, approximately 50° C./s or less is preferable. In addition, morepreferable cooling rate is less than 30° C./s.

In the present invention, the cooling rate described above indicates anaverage cooling rate in the above temperature range.

Primary objects of the finish annealing described above are to removestrain caused by rolling and to obtain a more appropriate crystal graindiameter by recrystallization for obtaining necessary iron lossproperties. The appropriate crystal grain diameter is generally in therange of approximately 20 to 200 μm as described above, and in order toobtain this crystal grain diameter, the temperature of the finishannealing is set to approximately 650° C. or more and preferably set toapproximately 700° C. or more. On the other hand, when the annealingtemperature is more than approximately 1,150° C., large and coarsegrains are formed, grain boundary cracking is liable to occur, anddegradation in iron loss is increased concomitant whit oxidation andnitridation of a steel sheet surface. Accordingly, the upper limit ispreferably set to approximately 1,150° C.

In the finish annealing, a holding time for the heating temperaturedescribed above is preferably set to 1 to 300 seconds.

A steel sheet manufactured in accordance with the conditions describedabove is a steel sheet having the features described in [Texture andProperties of Steel Sheet before Age-Hardening Treatment], a sufficientamount of the solute Cu, and small amount of large and coarse Cuprecipitates.

In addition, preferably, by age-hardening treatment at least at 500° C.for 10 hours, a steel sheet can be obtained having a strength not lessthan CYS (formula 1) or CTS (formula 3) described above and smalldecrease in iron loss.

The steel sheet of the present invention placed in this state has asmall yield strength (primarily depending on the Si content, when the Sicontents are 0.3% and 3.5%, the strengths are approximately 200 and 450MPa, respectively), and hence the punchabilities are superior.

The steel sheet described above is subsequently processed by agingtreatment. This aging treatment may be performed at any time, forexample, before coating and baking of an insulating film, after bakingthereof, or after machining such as punching. Of course, in view of thepunchabilities, it is preferable that shipping of the steel sheet beperformed before aging and that aging treatment be performed at acustomer site after punching; however, aging treatment may be performedin an optional step before shipping so that a steel sheet having a highstrength and a low iron loss is to be shipped.

In assembling a rotor using the non-oriented electrical steel sheet ofthe present invention, for example, aging treatment may be carried outfor punched non-oriented electrical steel sheet for laminating, orcarried out for laminated rotor core.

In aging treatment, even when the treatment condition is not limited tothe condition at 500° C. for 10 hours used as the index described above,as long as the following conditions are satisfied, distribution (averageparticle size and volume ratio) of the preferable fine Cu precipitatesdescribed above can be obtained. In addition, without seriouslydegrading the iron loss, strengths not less than CYS (formula 1) and CTS(formula 2) can be obtained.

The aging treatment is performed at a temperature in the range of fromapproximately 400 to 650° C. That is, when the temperature is less than400° C., precipitation of fine Cu becomes insufficient, and as a result,high strengths cannot be obtained. On the other hand, when thetemperature is more than 650° C., since large and coarse Cu precipitatesare formed, the iron loss is degraded, and the increase of strength isreduced. A more preferable temperature range is from approximately 450to 600° C. Although depending on the treatment temperature, a suitableaging time is from approximately 20 seconds to 1,000 hours andpreferably approximately 10 minutes to 1,000 hours.

EXAMPLES Example 1

Steel having the composition shown in Table 1 and containing the balancebeing iron and inevitable impurities was melted in a converter, followedby continuous casting, thereby forming a slab. Next, this slab wasformed into a hot-rolled steel sheet having a thickness of 2.2 mm by hotrolling and was then coiled at 500° C.

After this hot-rolled steel sheet was formed into a cold-rolled steelsheet having a final thickness of 0.5 mm by cold rolling, finalannealing was performed under the annealing conditions shown in Table 1.In this step, the average cooling rate from Ts calculated from theformula 2 to 400° C. was set to 20° C./s. In addition, the cooling ratein the range from 900° C. (annealing temperature for steel No. 8 and 10)to 400° C. was approximately equivalent to that described above.

Subsequently, an insulating film was formed. The composition of thesteel sheet thus obtained was the same as the slab composition shown inTable 1.

In addition to measurement of the average grain diameter d of the steelsheet (before aging), the iron loss W₁₅/W₅₀ (1), the punchabilities, theyield stress YS (1) were evaluated.

Next, after aging treatment was performed for the steel sheet at 500° C.for 10 hours, the properties after the aging treatment were evaluated bythe iron loss W₁₅/W₅₀ (2) and the yield stress YS (2). Furthermore, asample was obtained from the steel sheet, and the precipitate amount(volume ratio) of Cu precipitates and the average particle size thereofwere evaluated by observation using a scanning transmission electronmicroscope.

In this evaluation, as described above, the average crystal graindiameter d was obtained as the circle-base diameter by observation of across section of the steel sheet using an optical microscope. Inaddition, the iron loss was measured in accordance with JIS C2550 by anEpstein method using samples obtained along the rolling direction anddirection perpendicular thereto, the number of samples in the individualdirections being equal to each other. In addition, the punchabilitieswere measured by the number of ring-shaped samples (inside diameter of20 mm×outside diameter of 30 mm) punched out from the steel sheet atwhich a burr height thereof reached 30 μm. The yield strengths weremeasured along the rolling direction and the direction perpendicularthereto of the steel sheet using a tensile test (at a cross-head speedof 10 mm/min) and were averaged as the yield strength.

In addition, the evaluation of Cu precipitates was performed byobservation using a scanning transmission electron microscope asdescribed below. A sample in the form of a flat sheet for theobservation by an electron microscope was obtained from a centralportion of the steel sheet in the thickness direction, the flat sheetbeing parallel to the rolling direction, and was then processed byelectrolytic polishing using a peroxy acid-methanol base electrolyte toform a flat sheet having a smaller thickness. Next, for cleaning asurface of the sample thus obtained, sputtering was performed for 5minutes using argon ions for sample preparation. The observation wasperformed by a scanning transmission mode in which electron beams 1 nmor less in diameter was scanned in an observation field, and three darkfields per each were obtained in which the precipitates were easilyrecognized. When the observation region is too thin, a falling speed ofprecipitated particles is increased, and when the region is too thick,precipitated particles in the image of a scanning transmission electronmicroscope become difficult to recognize; hence, the thickness of thesample in the observation region was set in the range of from 30 to 60nm. In this measurement, the sample thickness was estimated from aspectrum of electron energy loss. For all the dark field images of 400nm by 400 nm thus obtained, particle recognition of Cu precipitates wasperformed by image processing, and the amount of precipitates wascalculated using the volume ratio of the volume of all precipitates tothe volume of the scope which was observed. In addition, from theaverage precipitate volume obtained from the volume of all precipitatesdivided by the number of recognized particles, the sphere-base diameterof the precipitates was obtained as the average particle size.

The evaluation results are shown in Table 2.

TABLE 1 Temperature of finish Composition (mass %) Ts annealing No. C SiMn Al P Ni Cu Others (° C.) (° C.) Remarks 1 0.002 2.5 0.10 0.20 0.020.01 0.1 — 510 1000 Comparative example 2 0.002 2.5 0.10 0.20 0.02 0.010.2 — 569 1000 Example 3 0.002 2.5 0.10 0.20 0.02 0.01 0.5 — 663 1000Example 4 0.002 2.5 0.10 0.20 0.02 0.01 1.5 — 807 1000 Example 5 0.0022.5 0.10 0.20 0.02 0.01 2.0 — 852 1000 Example 6 0.002 2.5 0.10 0.200.02 0.01 3.0 — 923 1000 Example 7 0.002 2.5 0.10 0.20 0.02 0.01 4.2 —989 1000 Comparative example 8 0.002 0.1 0.10 0.001 0.02 0.01 1.5 — 807820 Example 9 0.002 4.5 0.10 0.20 0.02 0.01 1.5 — 807 1000 Example 100.002 0.1 0.10 0.001 0.02 0.01 0.01 — 362 820 Comparative example 110.002 4.5 0.10 0.20 0.02 0.01 0.01 — 362 1000 Comparative example 120.002 2.5 3.0 0.20 0.02 0.01 1.5 — 807 1000 Example 13 0.002 2.5 0.103.0 0.02 0.01 1.5 — 807 1000 Example 14 0.002 2.5 0.10 0.20 0.50 0.011.5 — 807 1000 Example 15 0.002 2.5 0.10 0.20 0.02 5.0 1.5 — 807 900Example 16 0.002 2.5 0.10 0.20 0.02 0.002 1.5 Zr: 1 807 1000 Example 170.002 2.5 0.10 0.20 0.02 0.002 1.5 V: 1 807 1000 Example 18 0.002 2.50.10 0.20 0.02 0.002 1.5 Sb: 0.05 807 1000 Example 19 0.002 2.5 0.100.20 0.02 0.002 1.5 Sn: 0.05 807 1000 Example 20 0.002 2.5 0.10 0.200.02 0.002 1.5 Ge: 0.05 807 1000 Example 21 0.002 2.5 0.10 0.20 0.020.002 1.5 B: 0.005 807 1000 Example 22 0.002 2.5 0.10 0.20 0.02 0.0021.5 Ca: 0.005 807 1000 Example 23 0.002 2.5 0.10 0.20 0.02 0.002 1.5 Ce:0.005 807 1000 Example 24 0.002 2.5 0.10 0.20 0.02 0.002 1.5 Co: 0.5 8071000 Example 25 0.003 2.2 0.10 0.35 0.02 0.01 0.6 Zr: 0.12 684 1000Example V: 0.12 Ca: 0.002 26 0.002 2.2 0.10 0.35 0.02 0.01 0.6 Sb: 0.02684 1000 Example Sn: 0.03 B: 0.001 27 0.002 2.2 0.10 0.35 0.02 0.01 0.6Ge: 0.005 684 1000 Example Ce: 0.005 Co: 0.25

TABLE 2 Number Cu of Properties of Properties of precipitation Crystalpunching steel sheet steel sheet state grain (Ten before aging afteraging Change amount Volume diameter d thousand YS (1) W_(15/50) (1) CYSYS (2) W_(15/50) (2) ΔYS ΔW ratio Size No. (mm) times) (MPa) (W/kg)(MPa) (MPa) (W/kg) (2) − (1) (2) − (1) (vol %) (nm) Remarks 1 0.10 83385 2.7 520 420 2.7 35 0.0 0.02 9 Comparative example 2 0.10 81 365 2.5520 520 2.6 155 0.1 0.20 6 Example 3 0.10 89 370 2.5 520 612 2.7 242 0.20.41 6 Example 4 0.10 92 370 2.5 520 620 2.7 250 0.2 1.20 15 Example 50.10 86 374 2.4 520 608 2.6 234 0.2 1.34 18 Example 6 0.10 80 370 2.3520 522 2.6 152 0.3 1.40 20 Example 7 0.10 65 412 3.8 520 440 4.5 28 0.72.40 50 Comparative example 8 0.03 108 215 5.9 342 427 6.1 212 0.2 0.265 Example 9 0.10 65 550 2.0 710 850 2.2 300 0.2 1.34 18 Example 10 0.03103 206 6.0 342 225 6.1 19 0.1 0.00 — Comparative example 11 0.10 28 6102.2 710 612 2.2 2 0.0 0.00 — Comparative example 12 0.10 72 520 2.3 665670 2.8 150 0.5 1.20 12 Example 13 0.10 69 470 2.0 623 670 2.3 200 0.31.10 12 Example 14 0.10 65 565 2.4 728 780 2.7 215 0.3 1.25 15 Example15 0.10 85 495 2.2 644 680 2.6 185 0.4 0.90 7 Example 16 0.10 73 468 2.3520 620 2.5 152 0.2 1.00 18 Example 17 0.10 69 450 2.3 520 615 2.5 1650.2 1.10 15 Example 18 0.10 91 377 2.4 520 618 2.4 241 0.0 0.90 8Example 19 0.10 93 360 2.4 520 621 2.5 261 0.1 0.85 7 Example 20 0.10 85360 2.3 520 612 2.6 252 0.3 1.20 10 Example 21 0.10 80 365 2.5 520 6152.6 250 0.1 0.80 7 Example 22 0.10 93 354 2.5 520 613 2.6 259 0.1 1.20 8Example 23 0.10 85 370 2.5 520 605 2.6 235 0.1 1.40 9 Example 24 0.10 78409 2.3 520 607 2.5 198 0.2 1.20 12 Example 25 0.10 98 355 3.1 520 5703.3 215 0.2 0.60 8 Example 26 0.10 95 350 3.0 520 530 3.2 180 0.2 0.50 7Example 27 0.10 82 362 3.1 520 555 3.4 193 0.3 0.65 8 Example

As shown in Table 1, all steel sheets having the compositions controlledwithin the scope of the present invention had a high strength and asuperior iron loss after aging. In the steel of the present invention,the amount and the average particle size of Cu precipitates, whichfunctioned as the strengthening factors, were within the scope of thepresent invention. In addition, according to the steel of the presentinvention, by age-hardening treatment, the yield strength was increasedby 150 MPa or more, and in addition, the iron loss was decreased by 0.5W/kg or less.

In addition, the tensile strength of all the steel sheets of the presentinvention after aging was not less than CTS.

On the other hand, in conventional steel (comparative example: No. 10)having a low Si component and conventional steel (comparative example:No. 11) having a high Si component, although a superior iron loss couldbe obtained, the strength was inferior to that of steel of the presentinvention containing an equivalent amount of Si to that of the steelmentioned above. In addition, steel (comparative example: No. 7)containing excessive Cu had a poor iron loss before aging and a smallincrease in strength after aging as compared to steel of the presentinvention containing an equivalent amount of Si to that of theabove-mentioned steel.

Example 2

Steel having the composition shown in Table 3 was melted in a converter,followed by continuous casting, thereby forming a slab. In all the slabsthus obtained, the balance was iron and inevitable impurities.

Next, after the slab was formed into a hot-rolled steel sheet having athickness of 1.8 mm by hot rolling and was then coiled at 500° C.,hot-rolled sheet annealing was performed at 800° C. for 5 hours for thishot-rolled steel sheet thus obtained, and subsequently, by a single coldrolling method, a cold-rolled steel sheet having a thickness of 0.35 mmwas formed.

Furthermore, final annealing was performed for this cold-rolled steelsheet thus obtained under the annealing conditions shown in Table 4, andafter an insulating film is formed, aging treatment shown in Table 4 wasfurther performed. In this Table, the cooling rate was the averagecooling rate from Ts calculated from the formula 2 to 400° C.

The composition of the steel sheet was the same as the composition ofthe slab. In addition, the cooling rate in the range of from thetemperature of finish annealing to 400° C. was approximately equivalentto that shown in Table 4.

As was the case of Example 1, the average crystal grain diameter d, theiron losses W₁₅/W₅₀ and yield stress YS (MPa) before and after aging,and the amount (volume ratio) and the average particle size of Cuprecipitates after aging treatment were evaluated for the steel sheetsthus obtained. The results are shown in Table 4.

As shown in Table 4, in the steel sheets which were each controlled sothat the steel composition, the finish annealing conditions, and theaging conditions were within the scope of the present invention, theamount and the average particle size of the Cu precipitates were-withinthe specified range, and steel sheets (after aging) having a superioriron loss and a high strength could be obtained.

The steel sheets of the present invention all had a tensile strength notless than CTS after aging. In addition, in all the steel sheets of thepresent invention, by age-hardening treatment, the yield strength wasincreased by 150 MPa or more, ant the iron loss was decreased by 0.7W/kg or less.

However, in conventional steel b and d (comparative examples; Nos. 10and 19) which contained no Cu, although a superior iron loss could beobtained, a high strength by Cu precipitation can not be obtained.

In addition, when the temperature of finish annealing is too low(comparative examples: Nos. 1 and 11), since Cu in a solid solution formis not sufficiently formed in annealing, the amount of Cu precipitatesby aging became insufficient, and as a result, a high strength can notbe obtained. In addition, when the cooling rate of the finish annealingis too low (comparative examples: Nos. 4 and 14), since the size of Cuprecipitates was large, the iron loss was degraded, and in addition, ahigh strength can not be obtained.

Furthermore, when the aging temperature was too low (comparativeexamples: Nos. 5 and 15), since the amount of Cu precipitates wasinsufficient, a high strength could not be obtained, and when the agingtemperature was too high (comparative examples: Nos. 9 and 18), sincelarge and coarse Cu precipitates were considerably formed, the iron losswas degraded, and a high strength could not also be obtained.

TABLE 3 Steel Composition (mass %) Ts Classification of ID C Si Mn Al PNi Cu Others (° C.) components a 0.003 0.12 0.10 0.20 0.05 0.1 1.5 — 807Within scope of invention b 0.003 0.12 0.10 0.20 0.05 0.1 0.02 — 400 Outof scope of invention c 0.002 3.2 0.25 0.35 0.01 0.0 2.8 — 910 Withinscope of invention d 0.003 3.1 0.26 0.35 0.01 0.1 0.1 — 510 Out of scopeof invention

TABLE 4 Cu Properties of precipitation Finish annealing TemperatureCrystal steel sheet state Cooling of aging grain after aging VolumeSteel Ts Temperature rate treatment diameter CYS YS (2) W_(15/50) (2)ratio Size No. ID (° C.) (° C.) (° C./s) (° C.) (mm) (MPa) (MPa) (W/kg)(vol %) (nm) Remarks 1 a 807 800 10 500 0.025 384 314 6.7 0.15 15Comparative example 2 817 10 500 0.03 372 455 4.9 0.30 7 Example 3 85010 500 0.035 362 451 4.8 0.30 5 Example 4 817 5 500 0.03 372 310 6.50.50 25 Comparative example 5 817 10 350 0.03 372 258 4.9 0.01 3Comparative example 6 817 15 400 0.03 372 545 4.8 0.20 3 Example 7 81710 400 0.03 372 452 4.8 0.30 5 Example 8 817 10 650 0.03 372 440 4.81.90 18 Example 9 817 10 700 0.03 372 261 6.9 1.00 35 Comparativeexample 10 b 400 817 10 500 0.03 372 225 4.8 0.00 — Comparative example11 c 910 900 10 500 0.055 619 505 4.6 0.15 12 Comparative example 121000 10 500 0.13 586 595 2.6 1.80 13 Example 13 950 10 500 0.08 603 6402.6 1.70 12 Example 14 950 5 500 0.08 603 587 4.9 1.90 25 Comparativeexample 15 950 10 350 0.08 603 465 2.5 0.00 — Comparative example 16 95010 400 0.08 603 650 2.6 0.35 5 Example 17 950 10 650 0.08 603 610 2.91.90 17 Example 18 950 10 700 0.08 603 515 5.2 0.65 30 Comparativeexample 19 d 510 950 10 500 0.08 602 470 2.4 0.00 — Comparative example

Example 3

Steel slabs were prepared containing 3% of Si, 0.2% of Mn, and 0.3% ofAl as base components and containing various amounts of Cu and Ni. Thecompositions of the steel slabs are shown in Table 5, and the balancethereof was iron and inevitable impurities.

The slabs were each processed by hot rolling to form a sheet having athickness of 2.0 mm and were then coiled at 550° C. Subsequently,hot-rolled sheet annealing was performed at 1,000° C. for 300 seconds orwas not performed. Cooling after the hot-rolled sheet annealing wasperformed at an average cooling rate of 20° C./s in the range of from atleast Ts (obtained from the formula 2) to 400° C.

Subsequently, pickling and cold rolling for forming a steel sheet havinga finish sheet thickness of 0.35 mm were performed. Furthermore, afterfinish annealing was performed in which a holding temperature of 950° C.was maintained for 30 seconds, cooling was performed at a cooling rateof 6° C./s in a temperature range of from 900 to 400° C. The coolingrate in the range of from Ts to 400° C. was approximately equivalent tothat described above.

Next, after an insulating film was applied and baked, heating treatmentat 550° C. for 5 hours was performed for aging.

The average crystal grain diameter, the iron loss properties, and themechanical properties of the steel sheets thus obtained were evaluated.The compositions of the steel sheets were approximately equivalent tothose of the respective slabs. The iron loss was measured by an Epsteinmethod using samples obtained along the rolling direction and directionperpendicular thereto, the number of samples in the individualdirections being equal to each other. The mechanical properties weremeasured using samples obtained along the rolling direction and thedirection perpendicular thereto, and the evaluation was performed by theaverage value obtained therefrom. The details of the individualinvestigations were the same as those described in Example 1. Theresults are shown in Table 5.

In addition, as conventional electrical steel sheets formed to have ahigh tensile strength by known solid solution strengthening,grain-refining strengthening, precipitation strengthening, or the like,the following steel sheets were experimentally formed.

That is, as an example in that solid solution strengthening was used, asteel slab was hot-rolled and then processed by hot-rolled sheetannealing at 900° C. for 30 seconds, and warm rolling was then performedat 400° C. to form a steel sheet having a thickness of 0.35 mm, followedby finish annealing at 950° C. for 30 seconds. As shown in Table 6, thesteel slab described above contained 0.002% of C, 4.5% of Si, 0.2% ofMn, 0.01% of P, 0.6% of Al, 1.0% of W, 1.0% of Mo, and the balance beingiron and inevitable impurities.

In addition, as an example in that solid solution strengthening andgrain-refining strengthening were used, steel was hot-rolled and thencold-rolled to form a steel sheet having a thickness of 0.35 mm,followed by finish annealing at 800° C. for 30 seconds. As shown inTable 6, the steel described above contained 0.005% of C, 3% of Si, 0.2%of Mn, 0.05% of P, 4.5% of Ni, and the balance being iron and inevitableimpurities.

Furthermore, as an example in that precipitation strengthening bycarbides was used, steel was hot-rolled and then cold-rolled to form asteel sheet having a thickness of 0.35 mm, followed by finish annealingat 750° C. for 30 seconds. As shown in Table 6, the steel describedabove contained 0.03% of C, 3.2% of Si, 0.2% of Mn, 0.02% of P, 0.65% ofAl, 0.003% of N, 0.018% of Nb, 0.022% of Zr, and the balance being ironand inevitable impurities.

In all the examples described above, aging treatment was not performed.

TABLE 5 Crystal grain Properties of steel di- sheet after aging TS-Steel Steel composition (Mass percent) Ts ameter W_(15/50) B₅₀ TS CTSCTS No. ID C Si Mn P S Al Cu Ni N (° C.) (mm) (W/kg) (T) (MPa) (MPa)(MPa) Remarks 1 A 0.001 3.0 0.15 0.01 0.002 0.31 — — 0.003 — 0.083 2.451.69 501 601 −100 Com- parative example 2 B 0.002 3.01 0.18 0.02 0.0020.28 0.24 — 0.002 586 0.070 2.43 1.68 527 617 −90 Com- parative example3 C 0.003 3.2 0.21 0.01 0.003 0.28 1.2 — 0.002 774 0.085 3.46 1.68 681628 53 Com- parative example 4 D 0.003 3.14 0.2 0.02 0.002 0.32 3.8 —0.002 968 0.093 5.59 1.64 764 626 138 Com- parative example 5 E 0.0023.08 0.19 0.01 0.003 0.28 — 2.5 0.003 — 0.085 2.20 1.70 604 704 −100Com- parative example 6 F 0.002 3.06 0.18 0.02 0.002 0.29 0.11 1.0 0.002518 0.084 2.34 1.69 563 652 −90 Com- parative example 7 G 0.002 3.080.19 0.02 0.001 0.29 0.22 0.6 0.003 578 0.091 2.40 1.70 688 636 52Example 8 H 0.003 3.1 0.18 0.02 0.002 0.29 0.33 2.5 0.002 618 0.094 2.201.70 769 712 57 Example 9 I 0.002 3.04 0.21 0.01 0.003 0.3 1.1 1.2 0.002762 0.088 2.43 1.69 837 653 184 Example 10 J 0.002 3.06 0.2 0.02 0.0020.31 1.2 2.6 0.003 774 0.087 2.25 1.69 921 712 210 Example 11 K 0.0023.08 0.21 0.02 0.002 0.28 1.2 3.3 0.003 774 0.083 2.23 1.69 949 739 210Example 12 L 0.003 3.1 0.21 0.02 0.002 0.28 3.0 1.0 0.002 923 0.085 3.331.66 1009 660 349 Example 13 M 0.003 3.12 0.18 0.02 0.001 0.27 2.6 2.30.002 897 0.088 2.96 1.67 1053 708 345 Example 14 N 0.003 3.06 0.2 0.020.001 0.29 2.8 4.5 0.002 910 0.091 2.80 1.65 1164 784 379 Example

TABLE 6 Crystal Properties of steel grain sheet after aging Steel Steelcomposition (Mass percent) diameter W_(15/50) B₅₀ TS CTS TS-CTS No. ID CSi Mn P S Al Cu Ni N Others (mm) (W/kg) (T) (MPa) (MPa) (MPa) Remarks 15O 0.002 4.5 0.2 0.01 0.002 0.61 0 0 0.002 W: 1.0, 0.065 3.65 1.60 735769 −34 Conventional Mo: 1.0 example 16 P 0.005 3 0.2 0.05 0.003 0 0 4.50.002 — 0.041 5.90 1.66 688 819 −131 Conventional example 17 Q 0.03 3.20.2 0.02 0.003 0.65 0 0 0.003 Nb: 0.034 7.31 1.66 702 855 −153Conventional 0.016, example Zr: 0.017

Steel sheets Nos. 7 to 14 according to the present invention obtained asignificantly high strength while having superior magnetic propertiesapproximately equivalent to those of steel sheet No. 1 which was acomparative example having the base composition. Furthermore, even whenbeing compared to steel sheets Nos. 15 to 17, which were conventionalhigh-strength electrical steel sheets, the steel sheets described abovehad a significantly low iron loss or a high magnetic flux density, andthe compatibility of strength and magnetic properties was superior.

In addition, in all the steel sheets of the present invention, the yieldstress after aging was not less than CYS. In addition, according to allthe steel sheets of the present invention, the volume ratio of Cuprecipitates was in the range of from 0.3% to 1.9%, and the averageparticle size was in the range of from 1.5 to 20 nm. Furthermore, in thesteel sheets of the present invention, by age-hardening treatment, theyield strength was increased by 150 MPa or more, and the iron loss wasdecreased by 1.0 W/kg or less.

Example 4

Steel C of a comparative example and steel J of an example of thepresent invention shown in Table 5 were sequentially processed by hotrolling into a sheet having a thickness of 2.0 mm, hot-rolled sheetannealing at 1,000° C. for 300 seconds, cooling under the sameconditions as those in Example 3, pickling, and cold rolling into asheet having a finish sheet thickness of 0.35 mm. Furthermore, finishannealing was performed in which a holding temperature of 950° C. wasmaintained for 30 seconds, followed by cooling in a temperature range offrom 900 to 400° C. at an average cooling rate which was changed inaccordance with various conditions shown in Table 7. In this case, theaverage cooling rate in a temperature range of from Ts (obtained fromthe formula 2) to 400° C. was approximately equivalent to that describedabove.

Subsequently, an insulating film was applied and baked, thereby formingan annealed steel sheet. The annealed steel sheet thus obtained wasprocessed by heat treatment at 550° C. for 5 hours for aging. Theaverage crystal grain diameter, the iron loss, and the mechanicalproperties of the steel sheet thus obtained were evaluated. The detailsof the individual investigation were the same as those described inExample 1. In addition, the composition of the steel sheet wasapproximately equivalent to that of the corresponding slab.

The results are shown in Table 7 and FIGS. 2 and 3.

TABLE 7 Properties of Temperature Crystal Steel sheet after of finishHolding Cooling grain Aging aging Steel annealing time rate diametertemperature W_(15/50) B₅₀ TS CTS TS-CTS No. ID (° C.) (s) (° C./s) (mm)(° C.) (W/kg) (T) (MPa) (MPa) (MPa) Remarks 18 C 950 60 24 0.083 5502.74 1.68 812 629 184 Example 19 950 60 15 0.085 550 2.86 1.68 785 628158 Example 20 950 60 6 0.081 550 3.46 1.68 657 630 27 Comparativeexample 21 950 60 0.5 0.090 550 3.47 1.67 601 626 −26 Comparativeexample 22 J 950 60 24 0.094 550 2.25 1.7 970 709 262 Example 23 950 6015 0.092 550 2.25 1.69 945 709 236 Example 24 950 60 6 0.089 550 2.251.7 920 711 210 Example 25 950 60 2 0.085 550 2.39 1.7 896 712 184Example 26 950 60 0.5 0.088 550 3.04 1.7 738 711 53 Comparative example

As can be seen from the table and figures, steel C showed superiormagnetic properties and a high strength at a relatively high coolingrate (steel sheets Nos. 18 and 19) of 10° C./s or more; however, at acooling rate of less than 10° C./s, the iron loss was degraded, and thestrength was liable to decrease. On the other hand, in steel J of theexample containing an appropriate amount of Ni in addition to Cu, as canbe seen from the results of steel sheets Nos. 22 to 25, superiormagnetic properties and a high strength could be stably andsimultaneously obtained under various cooling-rate conditions.

In addition, the yield stress after aging of all the steel sheets of thepresent invention was not less than CYS. In addition, in all the steelsheets of the present invention, the volume ratio of Cu precipitates was0.6% to 1.2%, and the average particle size thereof was in the range offrom 5 to 15 nm. Furthermore, in all the steel sheets of the presentinvention, by age-hardening treatment, the yield strength was increasedby 190 MPa or more, and in addition, the iron loss was decreased by 0.4W/kg or less.

Example 5

Steel having the composition shown in Table 8 and the balance being ironand inevitable impurities was sequentially processed by hot rolling intoa sheet having a thickness of 2.0 mm, followed by hot-rolled sheetannealing for 300 seconds at a temperature shown in Table 9 or bynon-annealing. Subsequently, cooling under the same conditions as thosein Example 3 was performed, and pickling and cold rolling were thenperformed so as to form a sheet having a predetermined thickness.

Furthermore, finish annealing was performed in which a constanttemperature shown in Table 9 was maintained for 30 seconds, followed bycooling in a temperature range of from 900 to 400° C. at an averagecooling rate of 6° C./s. In this case, the average cooling rate in atemperature range of from Ts (obtained from the formula 2) to 400° C.was approximately equivalent to that described above.

Subsequently, an insulating film was applied and baked, thereby formingan annealed sheet. The annealed sheet thus obtained was processed byaging treatment at a temperature shown in Table 9 for 10 hours foraging.

The average crystal grain diameter, the iron loss, and the mechanicalproperties of the steel sheet thus obtained were evaluated. The resultsare also shown in Table 9. In addition, the composition of the steelsheet was approximately equivalent to that of the corresponding slab.From Table 9, it was found that all samples of individual steel sheetgrades have superior magnetic properties and significantly high strengthproperties.

In addition, the yield stress after aging of all the steel sheets of thepresent invention was not less than CYS. In addition, in the steelsheets of the present invention, the volume ratio of Cu precipitates was0.2% to 0.9%, and the average particle size thereof was in the range offrom 3 to 8 nm. Furthermore, in all the steel sheets of the presentinvention, by age-hardening treatment, the yield strength was increasedby 150 MPa or more, and in addition, the iron loss was decreased by 0.4W/kg or less.

TABLE 8 Steel Steel composition (Mass percent) No. ID C Si Mn P S Al CuNi N Others Remarks 26 R 0.003 0.35 0.15 0.15 0.002 0.001 0.55 1.1 0.003Example 27 S 0.002 1.50 0.18 0.02 0.002 0.28 1.5 1.0 0.002 Example 28 T0.003 4.11 0.21 0.01 0.003 0.28 1.0 1.1 0.002 Example 29 U 0.003 0.550.55 0.04 0.002 0.55 0.8 1.2 0.002 Example 30 V 0.002 3.08 0.19 0.010.003 1.1 0.8 2 0.003 Sb: 0.01 Example 31 W 0.002 3.06 0.18 0.02 0.0020.98 1.1 2.1 0.002 Sn: 0.05 Example 32 X 0.002 3.08 0.19 0.02 0.001 0.291.5 0.6 0.003 B: 0.002 Example 33 Y 0.003 3.10 0.18 0.02 0.002 0.29 0.332.5 0.002 Ca: 0.003 Example 34 Z 0.002 3.04 0.21 0.01 0.003 0.3 1.1 1.20.002 Co: 3.2 Example 35 e 0.001 3.05 0.15 0.01 0.001 0.31 1.5 1.5 0.001Zr: 0.13 Example V: 0.13 Ge: 0.003 La: 0.003

TABLE 9 Temperature of hot- rolled Temperature Crystal sheet Sheet offinish Cooling grain Aging After aging annealing thickness Ts annealingrate diameter temperature W_(15/50) B₅₀ TS CTS TS-CTS No. (° C.) (mm) (°C.) (° C.) (° C./s) (mm) (° C.) (W/kg) (T) (MPa) (MPa) (MPa) 26 — 0.5674 900 6 0.065 450 4.85 1.76 549 471 78 27 900 0.5 807 900 6 0.063 4503.64 1.75 749 527 222 28 1050 0.5 749 900 6 0.066 450 2.43 1.64 872 758115 29 950 0.5 720 1000 6 0.096 450 3.41 1.74 546 474 72 30 1050 0.2 7201000 6 0.096 500 2.06 1.69 828 739 89 31 1050 0.2 762 1000 6 0.113 5002.15 1.69 890 730 160 32 1050 0.2 807 1000 6 0.105 500 2.15 1.70 885 631254 33 1050 0.2 618 1000 6 0.109 500 1.97 1.71 757 707 50 34 1050 0.2762 1000 6 0.137 500 2.37 1.77 798 638 160 35 1050 0.2 807 1000 6 0.095500 3.85 1.69 911 656 254

INDUSTRIAL APPLICABILITY

According to the present invention, an age-hardenable non-orientedelectrical steel sheet can be obtained in which superior punchabilitiesand a superior iron loss can be simultaneously achieved and in whichstrengths are significantly increased by aging treatment.

In addition, according to the present invention, an electrical steelsheet having superior magnetic properties and high strengths can bestably provided.

From the steel sheets described above, rotors having high strengths andhigh reliability can be efficiently and economically manufactured, therotors being used for high speed motors and magnet-embedded type motors.

1. A non-oriented electrical steel sheet comprising: on a mass percent basis, C: 0.02% or less; Si: 4.5% or less; Mn: 3% or less; Al: 3% or less; P: 0.5% or less; Ni: 5% or less; and Cu: 0.2% to 4%, wherein a volume ratio of Cu precipitates in crystal grain interior is in the range of from 0.2% to 2%, and an average particle size of the Cu precipitates is in the range of from 1 to 20 nm.
 2. A non-oriented electrical steel sheet comprising: on a mass percent basis, C: 0.02% or less; Si: 4.5% or less; Mn: 3% or less; Al: 3% or less; P: 0.5% or less; Ni: 5% or less; and Cu: 0.2% to 4%, wherein the yield stress is not less than CYS (MPa) represented by the following formula 1, a volume ratio of Cu precipitates in crystal grain interior is in the range of from 0.2% to 2%, and an average particle size of the Cu precipitates is in the range of from 1 to 20 nm: CYS=180+5,600[% C]+95[% Si]+50[% Mn]+37[% Al]+435[% P]+25[% Ni]+22d ^(−1/2)  (Formula 1) where d is an average grain diameter (mm) of the crystal grains.
 3. A non-oriented electrical steel sheet comprising: on a mass percent basis, C: 0.02% or less; Si: 4.5% or less; Mn: 3% or less; Al: 3% or less; P: 0.5% or less; Ni: 5% or less; and Cu: 0.2%to 4%, wherein the steel sheet forms Cu precipitates in crystal grain interior having a volume ratio of 0.2% to 2% and an average particle size of 1 to 20 nm by aging treatment at 500° C. for 10 hours.
 4. The non-oriented electrical steel sheet according to one of claims 1 to 3, further comprising at least one of Zr, V, Sb, Sn, Ge, B, Ca, a rare earth element, and Co as a component, wherein the content of each of Zr and V is 0.1% to 3%, the content of each of Sb, Sn, and Ge is 0.002% to 0.5%, the content of each of B, Ca, and the rare earth element is 0.001% to 0.01%, and the content of Co is 0.2% to 5%.
 5. The non-oriented electrical steel sheet according to claim 1, wherein the steel sheet has an iron loss of 6 W/kg or less.
 6. The non-oriented electrical steel sheet according to claim 2, wherein the steel sheet has an iron loss of 6 W/kg or less.
 7. The non-oriented electrical steel sheet according to claim 3, wherein the steel sheet has an iron loss of 6 W/kg or less.
 8. The non-oriented electrical steel sheet according to claim 4, wherein the steel sheet has an iron loss of 6 W/kg or less.
 9. The non-oriented electrical steel sheet according to claim 1, wherein the steel sheet has a yield strength of 450 MPa or more.
 10. The non-oriented electrical steel sheet according to claim 2, wherein the steel sheet has a yield strength of 450 MPa or more.
 11. The non-oriented electrical steel sheet according to claim 3, wherein the steel sheet has a yield strength of 450 MPa or more.
 12. The non-oriented electrical steel sheet according to claim 4, wherein the steel sheet has a yield strength of 450 MPa or more. 